Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement or ion-optical arrangement
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/244—Detectors; Associated components or circuits therefor
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/30—Electron-beam or ion-beam tubes for localised treatment of objects
  • H01J37/317—Electron-beam or ion-beam tubes for localised treatment of objects for changing properties of the objects or for applying thin layers thereon, e.g. for ion implantation
  • H01J37/3171—Electron-beam or ion-beam tubes for localised treatment of objects for changing properties of the objects or for applying thin layers thereon, e.g. for ion implantation for ion implantation
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/244—Detection characterized by the detecting means
  • H01J2237/24405—Faraday cages
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/244—Detection characterized by the detecting means
  • H01J2237/2446—Position sensitive detectors
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/244—Detection characterized by the detecting means
  • H01J2237/2449—Detector devices with moving charges in electric or magnetic fields
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/245—Detection characterised by the variable being measured
  • H01J2237/24507—Intensity, dose or other characteristics of particle beams or electromagnetic radiation

Definitions

• This invention relates to ion implantation of semiconductor wafers and, more particularly, to a system for ion dose measurement and uniformity monitoring in an ion implantation system.
• Ion implantation has become a standard technique for introducing impurities into semiconductor wafers.
• the impurities determine the conductivity of the region into which they are implanted.
• Impurities are introduced into the bulk of semiconductor wafers by using the momentum of energetic ions as a means of imbedding them into the crystalline lattice of the semiconductor material.
• Ion implantation systems typically include an ion source for converting a gas or a solid material into a well-defined ion beam.
• the beam is mass analyzed to eliminate undesired ion species, is accelerated to the desired energy and is focused onto a target plane.
• the beam is deflected over the target area by beam scanning, by target movement, or a combination of scanning and target movement.
• One form of beam scanning utilizes two-dimensional electrostatic scanning over the target area utilizing a raster scan (see, for example, U.S. Patent No. 4,283,631) .
• ion implants are specified in terms of ion species/ ion energy and dosage in ions per square centimeter. Continuous measurement of ion dosage is necessary, since ion sources do not deliver accurate, constant ion beam currents. It is further necessary to monitor the spatial uniformity of the implanted dose
• Small Faraday cups are located at the four corners of the mask and sense the beam current at these locations. Individual conductors connect the four corner cups to a monitoring system which determines the deviation of the beam current at each corner from an average value. In some systems, the corner cups have been connected in common for measurement of cumulative ion dose.
• the apparatus comprises means for sensing the ion beam at a plurality of different sensing locations and for providing a beam signal representative of the ion beam current received at each of the sensing locations, means for integrating the beam signal
• uniformity monitoring means responsive to the scanning signal and to the beam signal for determining the beam current component received at each of the different sensing locations and determining ion dose uniformity therefrom.
• the sensing means includes a mask assembly positioned in the path of the ion beam in a plane substantially perpendicular to the beam.
• the mask assembly includes a mask plate having a first aperture for passage of the ion beam to the target plane and a plurality of beam sensing apertures positioned at each of the sensing locations around the first aperture, and an annular Faraday cup positioned behind the mask plate in alignment with the sensing apertures for sensing the beam current received through each of the sensing apertures.
• a single beam current signal is provided to the uniformity monitoring means, which includes demultiplexing means responsive to the scanning signal for identifying the sensing location of each beam current component and for switching the beam signal in response to the determination.
• the demultiplexer provides the capability to simultaneously measure ion dose and ion dose uniformity with the same sensor assembly.
• FIG. 1 is a simplified block diagram of an ion implantation system in accordance with the present invention
• Fig. 2 illustrates the mask assembly of the system shown in Fig.l as viewed along the ion beam axis;
• Fig. 3 is a cross-sectional view of the mask assembly shown in Fig. 2 taken along the line 3-3;
• Fig. 4 is a block diagram of the dose processor and uniformity monitor shown in Fig. 1;
• Fig. 5 is a block diagram of the demultiplexer shown in Fig. 4.
• Fig. 6 is a graphical representation of voltage waveforms in the demultiplexer of Fig. 5.
• FIG. 1 A simplified diagram of an ion implantation system including the apparatus of the present invention is shown in Fig. 1.
• An ion source 10 An ion source 10
• Ion source 10 converts a gas of selected species into a well-defined ion beam 12.
• the ion beam 12 passes through ion optics 14 which include an analyzer magnet for selecting a desired species, beam defining slits, an accelerating tube and a lens for focusing the beam in a target plane 15.
• the system utilizes an electrostatic deflection system including x-deflection plates 16 and
• _y-deflection plates 18 to scan the beam 12 over the target plane 15.
• the waveforms for deflecting the beam 12 are generated in a scan controller 20 for energizing the plates 16, 18 to achieve a desired scanning pattern.
• a semiconductor wafer 22 is positioned for processing in the target plane 15 in the path of the ion beam 12 and is mounted on a support platen 26.
• a beam sensing assembly 30 is positioned in the path of the beam 12 between the deflection plates 16 and the wafer 22.
• the assembly 30 provides a beam signal I ⁇ representative of ion beam current to a dose processor and uniformity monitor assembly 32. Signals x-scan and y-scan, corresponding to the deflection signals supplied to deflection plates 16, 18, are supplied to assembly 32.
• the function of the beam sensing assembly 30 and the dose processor are supplied to assembly 32.
• a typical ion implantation system includes dual target chambers and an automatic wafer handling system for transferring semiconductor wafers through vacuum locks for introduction into the target chambers.
• the wafer handling system properly locates, aligns and cools the wafers during processing and removes the processed wafer from the chamber at the conclusion of processing. Such wafer handling and cooling is not within the scope of the present invention.
• the beam sensing assembly 30 is shown as viewed along the beam axis in Fig. 2 and in cross-section in Fig. 3.
• a mask plate 36 includes mounting holes 38 for mounting the assembly 30 in the path of the ion beam 12.
• the mask plate 36 is provided with a relatively large circular aperture 40 for passage of the beam 12 to the wafer 22.
• the aperture 40 must be large enough to avoid shadowing the wafer 22.
• An x-y coordinate system having an origin at the center of the aperture 40 is shown in Fig. 2.
• the beam 12 passes along a z axis (Fig. 1) through the origin of the x-y plane and is offset by a fixed angle, typically seven degrees (not shown), to remove neutral particles from the beam 12.
• Sensing apertures 42, 43 r 44, 45 are provided in the mask plate 36 around the periphery of the aperture 40.
• the sensing apertures 42, 43, 44, 45 are equally spaced and are each oriented at 45° with respect to the x-y axis.
• Each of the sensing apertures 42, 43, 44, 45 leads to a beam current sensor.
• the ion beam 12 is raster scanned over the surface of the mask plate 36 in a generally rectangular pattern. The majority of the beam 12 passes through the aperture 40 and is implanted in the wafer 22. When the beam 12 passes over the sensing apertures 42, 43, 44, 45, it is measured as described hereinafter.
• FIG. 3 A detailed cross-sectional view of the beam sensing assembly 30 is shown in Fig. 3.
• An aperture plate 48 typically of graphite, is mounted to the rear of the mask plate 36 and accurately defines the dimensions of the sensing apertures 42, 43, 44, 45.
• Graphite resists sputtering by the ion beam 12.
• first bias electrode 50 and a second bias electrode 52.
• annular Faraday cup 54 Mounted behind the electrodes 50, 52 is an annular Faraday cup 54.
• the electrodes 50, 52 and the Faraday cup 54 are contained within an annular cover or housing 56.
• the electrodes 50, 52 and the Faraday cup 54 are insulated from each other and from ground by a series of insulated standoffs
• Electrode 50 is typically biased at about -300 volts, and electrode 52 is typically biased at about -200 volts. Electrodes 50, 52 function to contain secondary electrons generated within the Faraday cup 54.
• Faraday cup 54 is an annular conductive ring coaxial with the aperture 40 and of larger diameter than aperture 40. It has four beam collection holes 55 partially through the thickness of the ring and aligned with apertures 42, 43, 44, 45. The holes 55 collect ions passing though the sensing apertures 42, 43, 44, 45.
• Faraday cup 54 has a single electrical connection to the dose processor and uniformity monitor assembly 32 as shown in Fig. 1. A block diagram of the dose processor and uniformity monitor assembly 32 is shown in Fig. 4.
• the beam current signal I ⁇ from the annular Faraday cup 54 is supplied to a current to voltage converter 60 which can be a resistor or an operational amplifier. Pulses of beam current are received by the Faraday cup 54. The amplitude and duration of the pulses depends on beam current, area of the aperture, the scan speed and whether the beam 12 passes directly across the center of the aperture or across an edge thereof. Since the geometry of the apertures and the scan characteristics are known, the beam current is the only variable.
• the output voltage V ⁇ of the converter 60 is a pulse train as shown in Fig. 6, which represents the beam current received by all four apertures, 42, 43, 44, 45.
• the voltage V ⁇ is supplied to a voltage to frequency converter which is typically a voltage controlled oscillator 62 operating in the range between 0Hz and 9400Hz.
• the oscillator 62 includes at its input a low pass filter which takes the average value of the voltage V ⁇ .
• the output of the oscillator 62 is a frequency dependent upon ion beam current.
• the output of the oscillator 62 is accumulated in a digital counter 64 during the implant.
• the number stored in counter 64 represents the cumulative ion dosage during the time to implant a wafer. When a prescribed count representing a desired dosage is reached, the implant is terminated.
• the output voltage V ⁇ of the converter 60 is also supplied to a demultiplexer 70 which receives x-scan and y-scan signals from the scan controller 20 and demultiplexes the voltage V ⁇ into separate components V ⁇ 42 , V ⁇ 43 , V ⁇ 44 , V ⁇ 45 representing the beam current received at the sensing apertures 42, 43, 44, 45, respectively.
• These individual voltages are supplied to voltage-to-frequency converters such as voltage controlled oscillators 74 which generate an output frequency dependent on the voltage input.
• the oscillators 74 at their inputs include low pass filters which take the average value of the voltages
• the outputs D 42 / D 43# D 44' D 45 represent the cumulative dosage at each of the corner cups.
• the values of dosage at each corner cup are compared with an average dosage value and deviations are determined. The deviations from the average value are a measure of the uniformity of the implant.
• FIG. 5 A schematic diagram of the demultiplexer 70 is shown in Fig. 5.
• the x-scan and y-scan signals are supplied to buffer amplifiers 80, 82, each comprising an operational amplifier with the output connected to the inverting input.
• the outputs of the buffer amplifiers 80, 82 are supplied to detector circuits 84, 8.6, respectively, which detect the average DC component of the input waveform.
• Fig. 6 there is shown a graphic representation of the x-scan signal.
• the x-scan signal is a periodic waveform comprising successive positive and negative ramp voltages which sweep the ion beam across target plane.
• the x-scan signal typically has a non-zero average voltage V AV « When the instantaneous value of the x-scan signal is greater than V AV , the beam is located in the right half of the x-y plane, while values of the x-scan signal less than V... deflect the ion beam to the left half of the x-y plane.
• the y-scan signal has a similar waveform but a substantially longer period, since many x axis ' scans are completed during one vertical sweep of the target plane.
• the x-scan and y-scan signals are similar to raster scan signals used in cathode ray tube technology.
• Each of the detectors 84, 86 includes an input resistor 90' connected to the noninverting input of an operational amplifier 92.
• a capacitor 96 is connected between the noninverting input of each operational amplifier 92 and ground.
• The. output of each operational amplifier 92 is connected to the inverting input and represents the average DC component of the input signal.
• the output of the detector 84 is supplied through a resistor 94 to the non-inverting input of a comparator 100.
• the output of the buffer amplifier 80 is supplied to. the inverting input of the comparator 100.
• the output of the detector 86 is supplied through a resistor 94 to the non-inverting input of the comparator 102, while the output of the buffer amplifier 82 is supplied to the inverting input of the comparator
• a resistor 98 is coupled between the output of each of the comparators 100, 102 and the
• Each resistor pair 94, 98 provides hysteresis in the comparator operation.
• the comparators 100, 102 compare the instantaneous value of the x-scan and y-scan signals with the respective average values and provide an output signal of one state when the instantaneous value exceeds the average value and a second output state when the instantaneous value does not exceed the average value.
• the outputs of comparators 100, 102 are supplied to a binary decoder 104.
• the decoder 104 decodes the possible output states of the comparators 100, 102 into an active signal on one of four output lines depending on the location of the ion beam 12.
• the demultiplexer circuit shown in Fig. 5 and described above determines the aperture upon which the ion beam 12 is incident at each instant of time. This information permits the composite beam signal V ⁇ to be broken down, or demultiplexed, into the components received from each aperture.
• the outputs of the decoder 104 are used to control analog switches 110, 112, 114, 116 which can be field effect transistor analog switches.
• the voltage V ⁇ is applied to a buffer amplifier 120 having its output connected to the analog input of each of the analog switches 110, 112, 114, 116.
• the buffer amplifier 120 can comprise an operational amplifier having its output connected to its inverting input with the input signal received at the non-inverting input.
• the outputs of the analog switches 110, 112, 114, 116 represent the portion of the beam signal V ⁇ received at each individual aperture 42, 43, 44, 45.
• the composite beam signal V ⁇ has been demultiplexed into its individual components V ⁇ 42 , B43 , V ⁇ 44 , and ⁇ 45 .
• the individual beam current components from each of the apertures are processed to determine deviation from an average value which indicates uniformity.
• the voltage V ⁇ from the converter 60 includes pulses of varying amplitude and duration received through both of the apertures 42, 45 as described above.
• the demultiplexer separates the voltage V ⁇ into its components. The voltage V ⁇ pulses occuring during the time when the x-scan signal is less than V, .
• Similar demultiplexing of the " voltage V ⁇ is performed when the beam 12 is scanning over the apertures 43, 44.
• a wafer 22 is mounted in the target plane 15.
• the beam 12 is switched from a beam dump onto the wafer 22 and is scanned by the ' x-scan and y-scan signals over the surface area continuously.
• the beam 12 is overscanned beyond the edges of the wafer 22 by a sufficient amount to cover the apertures 42, 43, 44, 45.
• the system shown and : described hereinabove continuously measures cumulative dose by accumulating the beam current signals received at all of the apertures 42, 43, 44, 45.
• the system monitors ion dose uniformity by individually monitoring the dose
• the dose measurement and uniformity monitoring system of the present invention simplifies construction and improves reliability in comparison with prior art systems. As a result ion implantation systems are easier to manufacture and are lower in cost.
• the target wafer can be grounded since it is not part of the Faraday system.
• the present invention permits the ion beam to reach the wafer without passing through secondary electron suppression fields which can adversely affect dose uniformity, since the fields are localized in the region of the corner Faraday cups.

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• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Electron Sources, Ion Sources (AREA)
• Measurement Of Radiation (AREA)
• Physical Vapour Deposition (AREA)

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• 1986
  • 1986-05-16 US US06/864,584 patent/US4751393A/en not_active Expired - Lifetime
• 1987
  • 1987-04-27 JP JP62502889A patent/JPH0628141B2/ja not_active Expired - Fee Related
  • 1987-04-27 WO PCT/US1987/000985 patent/WO1987007076A1/en active IP Right Grant
  • 1987-04-27 EP EP87903184A patent/EP0276229B1/en not_active Expired - Lifetime
  • 1987-04-27 DE DE3789103T patent/DE3789103T2/de not_active Expired - Fee Related
  • 1987-05-15 KR KR1019870004807A patent/KR950001252B1/ko not_active IP Right Cessation

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